Reliability-related issues of integrated circuits have existed for decades. In the past, bias temperature instability (BTI, often known for negative bias temperature instability (NBTI) when related to PMOS devices) related issues have had a significant impact on designs of integrated circuits. To predict the behavior and the reliability of integrated circuits, simulation program with integrated circuit emphasis (SPICE) aging models need to be established, which requires BTI tests to be performed in order to collect data from integrated circuit devices.
The most commonly used parameters in the BTI tests are saturation drain current (Idsat) degradation ΔIdsat, linear drain current (Idlin) degradation ΔIdlin, and threshold voltage (Vt) degradation ΔVt, which are indications as to how devices degrade with high-temperature stress and voltage stress. FIG. 1 illustrates a conventional scheme for measuring one of the parameters, for example, ΔIdsat. The Y-axis represents the BTI degradation, which may be the percentage of the Idsat reduction caused by stresses applied to a device. The X-axis represents the stress time. After each of the stresses, the parameter of interest is measured. It is noted that at the time the parameter is measured, the device recovers partially from the stress (note the drop of the dots), and hence the Idsat degradation ΔIdsat is reduced. Using the method illustrated in FIG. 1, only one parameter, such as Idsat degradation ΔIdsat, can be measured.
To establish the aging model, however, ΔIdsat, ΔIdlin, and ΔVt are all required. Therefore, all of these three parameters need to be measured, preferably from a same device since the three parameters are highly correlated. FIG. 2 illustrates a second methodology for measuring the three parameters. After a period of stress time, ΔIdsat is measured, followed by the measurement of other parameters. In the illustrated example, an I-V sweep is performed, so that ΔIdlin and ΔVt can be extracted. However, it is noted that during the time ΔIdsat, ΔIdlin, and ΔVt are measured, the device continuously recovers from the previous stress. Therefore, the value of ΔIdsat actually represents a different degradation value of the device than the values of ΔIdlin and ΔVt. For example, at the time ΔIdsat is measured, the device may be degraded by 5 percent, while when ΔIdlin and ΔVt are measured, the device has recovered to a degradation level of only 4 percent. Therefore, the measured ΔIdsat, ΔIdlin, and ΔVt values cannot reflect the real correlation between these parameters. The incorrectly reflected correlation will adversely affect the SPICE aging model established from these measurement results.
The results obtained using the method shown in FIG. 2 are shown in FIG. 3, which illustrates ΔIdsat and ΔIdlin as functions of stress time. FIG. 3 reveals several drawbacks in the conventional BTI test methodology. First, it is found that circles, which are measured ΔIdsat values, are greater than the squares, which are measured ΔIdlin values. These results conflict with the practical situation, in which the ΔIdsat values should always be less than the ΔIdlin values. Second, the slope (n) of ΔIdsat is 0.13, which is significantly different from the slope of ΔIdlin (0.17). This also conflicts with the practical situation, in which the slopes of the ΔIdsat values and the ΔIdlin values should be substantially the same.
In FIG. 3, the incorrect result that ΔIdsat values are greater than the ΔIdlin is a consequence of measuring ΔIdsat values before measuring ΔIdlin and ΔVt values. If, however, ΔIdlin and ΔVt values are measured before ΔIdsat, ΔIdsat values will be less than the ΔIdlin values. This indicates that the measurement errors have dominated the measurement in the conventional BTI test methodology.
FIG. 4 illustrates threshold voltage degradation ΔVt as a function of stress time. It is noticed that the extracted slope is 0.217, which is also significantly different from the slopes of ΔIdsat and ΔIdlin. Again, this conflicts with the real situation, in which the slope of ΔVt should be substantially equal to the slopes of ΔIdsat and ΔIdlin. A new measurement methodology is thus required to overcome the above-discussed problems and provide a more accurate modeling.